The present invention relates to paper coaters generally, and in particular to an improved dip roll coating system, in which film split and streaking patterns in a coating applied onto a paper web are substantially eliminated.
Paper coating processes continue to be performed at faster web speeds to increase productivity. At the same time, paper quality continues to rise because of printer demands, necessitating increasingly higher coat weights to produce paper of the required quality.
The standard paper coating system for high cost weights and high web speeds has been the dip roll coater. Essentially, to apply a high weight of coating material onto the surface of a paper web traveling at a high rate of speed, for example at 2500 fpm and faster, a dip roll applicator applies an excess of coating onto the web as it is carried through a nip between the dip roll and a backing roll and, downstream from the dip roll, the coating is doctored on the web by a blade. At high web speeds, a dip roll produces a severe film split pattern in the excess layer of coating applied onto the web, i.e., separations or thin areas occur in the coating, extending along the direction of web travel. When the nonuniform layer of excess coating is doctored by the blade, it exerts varying impulse forces against the blade, and is doctored nonuniformly. In consequence, the film split pattern is not removed by the blade and appears as narrow machine direction banding in the finished sheet, and the higher the coat weight and faster the web speed, the more pronounced is the film split pattern in the finished sheet.
A dip roll coater also is commonly used to apply high coat weights at somewhat slower web speeds, for example on the order of 1200-1600 fpm. In this case, a backing roll often is not used to define with the dip roll a nip through which the web passes, but instead a "kiss coater" arrangement may be used, in which the web is simply carried under tension across the dip roll surface for receiving an excess of coating from the surface. The dip roll usually is rotated so that its surface moves in the same direction as but slower than web travel, and downstream from the roll the excess coating is doctored on the web by a blade. A problem encountered is that coating picked up on the dip roll surface for transfer to the web is not uniform and level, but instead is wavy and defines crests and troughs. Absent a backing roll for pressing the web against the dip roll, the web tends to ride on the crests as it is carried across the dip roll, so the excess coating layer is transferred to the web is nonuniform and blotchy. When the nonuniform and blotchy layer of coating is doctored by the blade, it is doctored nonuniformly, and a streaking pattern occurs in the coating on the finished paper sheet.